The present invention relates to an MIM (metal-insulator-metal) device which is useful as a switching device and, in particular, a switching device for use in liquid crystal display.
As a typical example of thin film 2-terminal device, an MIM device may be exemplified. Conventional MIM devices are disclosed in, for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 62.62333 (1987) and 61.260129 (1986).
Specifically, Japanese Patent Application Laid-Open (KOKAI) No. 62.62333 (1987) discloses an MIM device comprising a tantalum (Ta) thin film formed by sputtering as a lower electrode, a tantalum oxide (Ta.sub.2 O.sub.5) film formed by anodic oxidizing tantalum of the lower electrode as an insulator, and a Cr film formed by vapor deposition, or indium - tin oxide (ITO) film formed by further applying ion-plating vapor deposition, that is, Cr/ITO film as an upper electrode.
Japanese Patent Application Laid-Open (KOKAI) Nos. 61-260219 (1986) discloses a non-linear resistance device comprising an indium-tin oxide (ITO) film formed by magnetron sputtering as a lower electrode, an amorphous film mainly composed of silicon such as a silicon oxide film or a silicon nitride (SiNx) film formed by plasma CVD as an insulator, and a chromium film or a metal film of Al, Cu, NiCu, Ag or Au formed by sputtering as an upper electrode.
These MIM devices are utilized, in particular, as switching devices for use in liquid crystal display of active matrix system. However, the MIM device disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-62333 (1987) has the following drawbacks:
(1) Since the insulator is formed by the anodic oxidation of the lower electrode metal film, it is difficult to prepare a film having desired physical properties and thus, the control for the device characteristics is not easy, and accordingly the degree of freedom for the device design is restricted.
(2) In the case of using the device for a liquid crystal display, since the rubbing treatment for orienting liquid crystals in a predetermined direction is necessary, an insulator layer of large thickness or high hardness, for example, Vickers hardness of not less than about 2000 kg/mm.sup.2 is required so as to be free from mechanical damages for the film or the device. However, the anodic-oxidized film is soft and the film thickness has to be kept not less than about 600 .ANG. in view of the current-voltage characteristic and driving voltage.
(3) In the case of the anodic-oxidized film, the heat treatment at about 300.degree.-500.degree. C. is necessary in order to obtain symmetrical polarity. So, there is imposed a restriction on the material of the substrate.
(4) In a case of using the device for a liquid crystal, since a ratio of capacitance in liquid crystal portion (C.sub.LCD) to device capacitance (C.sub.MIM) of not smaller than about 10/1 is necessary, the lower dielectric constant of the insulator is more advantageous for fabrication. However, since anodic-oxidized film such as Ta.sub.2 O.sub.5 has high dielectric constant (about 25 in the case of Ta.sub.2 O.sub.5), it requires highly fine fabrication technique and creates a difficulty as to the preparation of the device with a large area at high yield.
On the other hand, the MIM device as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 61-260219 (1986) can overcome the foregoing drawbacks (1), (2) and (4), but it also involves the drawback as in (3) above since the film-deposition temperature is as high as about 300.degree. C. Moreover, it also has the following drawbacks.
(5) In the case of preparing a large area device, the film thickness and film quality tend to become non-uniform because of the variation of the temperature distribution on a substrate and accordingly, it is not suitable as a thin film device.
(6) Further, since a lot of pin-holes are formed due to dusts generated in a gas phase upon film deposition, the production yield of the device is lowered, or peeling is liable to occur because of large film-stresses which also causes the reduction of the yield.
As a result of present inventors' earnest studies for solving the above mentioned problems in the conventional MIM device , it has been found that an MIM device having a great degree of freedom in view of device design, having high mechanical strength, and being free from restriction as to the material of a substrate and suitable to thin film device as well as for ease of preparation of larger area and of improved with film properties such as hardness, stability, etc. can be obtained by using in an MIM device comprising a lower electrode, an upper electrode and an insulator sandwiched therebetween as an insulator layer, a hard carbon film containing as the constituent element, carbon atom, hydrogen atom and at least one of elements selected from the group consisting of group III element, group IV element and group V element of the periodical table, alkali metal element, alkaline earth metal element, nitrogen atom, oxygen atom, chalcogen element and halogen atom. The present invention has been accomplished based on this finding.